5. Central Air Conditioners

5. Central Air Conditioners

Central air conditioners are designed to cool the entire house. The large compressor
and outdoor coil are located outdoors and are connected by refrigerant lines
to an indoor coil mounted in the furnace (Figure 6). The same duct system is used for both heating
and cooling air distribution.

Figure 6 Installed central air conditioner

How does a central air conditioner
work?

A central air conditioner uses energy to take heat away. The most common type
uses a compressor cycle (like a refrigerator), illustrated in Figure7, to transfer heat from the house to the outdoors.
Using a special fluid called a refrigerant, heat is absorbed and released when
the refrigerant changes back and forth between a liquid and gas state. As it
changes from liquid to gas, it absorbs heat; in changing back to a liquid from
a gas, it releases heat.

The compressor cycle passes liquid refrigerant through an expansion device,
changing the liquid to a low-pressure liquid/gas mixture. In the indoor coil
or evaporator, the remaining liquid absorbs heat from household air and becomes
a low-temperature gas.

Figure 7 The operation of a central air conditioner

The low-temperature gas is compressed by a compressor that reduces its volume
and increases its temperature, causing it to become a high-pressure, high-temperature
vapour. This vapour is sent to the outdoor coil or condenser where its heat
is transferred to the outdoor air, causing the refrigerant to condense into
a liquid. The liquid returns to the expansion device and the cycle is repeated.

Household air is cooled and dehumidified as it passes over the indoor coil.
The moisture removed from the air, when it contacts the indoor coil, is collected
in a pan at the bottom of the coil and sent to a house drain.

Energy efficiency considerations

Select a central air conditioner with as high a SEER as is practical within
your budget. The annual cooling efficiency of a central air conditioner is affected
by the manufacturer's choice of features and components. The SEER of central
air conditioners ranges from a minimum of 10.0 to a maximum of about 17.0.

An ENERGY STAR® qualified central air conditioner must have a SEER
rating of at least 12 or greater for a single-package unit and 13.0 or greater
for a split system.

More efficient compressors, larger and more effective heat exchanger surfaces,
improved refrigerant flow and other features are largely responsible for recent
improvements in the efficiency of central air conditioners.

Figure 8 Efficiency of a central air conditioner

Advanced reciprocating, scroll and variable-speed or two-speed compressors,
when combined with the current best heat exchangers and controls, permit SEERs
as high as 17.0 (Figure 8). Central air conditioners with
the highest SEERs always use variable-speed or two-speed high-efficiency compressors.

Sound considerations

Select a central air conditioner with an outdoor sound rating of about 7.6
B or lower, if possible. The sound rating is expressed in bels. The lower the
sound rating, the lower the sound power emitted by the outdoor unit. New, energy-efficient
designs often have low sound ratings. The ratings are published by the Air-Conditioning
and Refrigeration Institute (ARI), 4301 North Fairfax Drive, Arlington, Virginia,
22203 U.S.A.

Sizing considerations

Cooling loads should be determined by a qualified air-conditioning contractor,
using a recognized sizing method such as that specified in CSA-F280-M90: Determining
the Required Capacity of Residential Space Heating and Cooling Appliances.
Do not rely on simple rules of thumb for sizing, but insist on a thorough analysis
from the sales representative.

Select a central air conditioner size or capacity to just meet the design cooling-load
calculated. Over sizing the unit will result in short operating cycles, which
will not adequately remove humidity, resulting in an unpleasantly cold and damp
home. Under sizing the unit will result in an inability to attain a comfortable
temperature on the hottest days.

Also, with a central air conditioning system, the equipment cost is much more
proportional to size than it is with heating equipment. Unnecessary over sizing
will increase the purchase price and increase on-and-off cycling, which will
decrease the unit's overall efficiency.

Installation considerations

When installing a central air conditioner, it is important that the contractor
follow the manufacturer's instructions carefully.

The outside compressor for a central air conditioner should be shaded from
direct sun.

The following general guidelines should be considered when installing a central
air conditioner:

Locate the outdoor unit or condenser in a cool, shaded place where the waste
heat can be readily rejected.

Locate the outdoor unit where its noise will not be a problem for you or
your neighbour. This generally means away from bedroom windows or patios and
not between houses.

In new construction, consider installing the central air conditioner outdoor
unit on a frame mounted to the house. This avoids problems due to settlement
of backfill around the foundation, which causes the outdoor unit to lose its
level.

The central air conditioner will generally require more airflow than the
furnace needs for heating. Consider a two-speed fan motor with the correct
speed automatically selected depending on whether cooling or heating is called
for.

Keep refrigerant lines as short as possible, and where the lines pass through
the outside wall, ensure that the surrounding space between the lines and
the wall is packed with a resilient material, such as plumber's putty. This
will prevent noise or vibration problems and air leaks.

The cost of installing a central air conditioner will vary depending on the
nature of the existing furnace, whether or not the existing ductwork needs to
be modified, and whether there is a need to upgrade the electrical service to
deal with the increased electrical load of the central air conditioner.

Where an existing central air conditioner is being replaced, ensure that the
existing indoor coil is replaced by one matched to the new outdoor unit. If
the existing indoor coil is not replaced, the new unit will not deliver its
rated efficiency.

Operation considerations

In the interest of energy efficiency, use central air conditioning only when
ventilation is inadequate to ensure comfortable conditions. Natural ventilation
of the house at night, when it is relatively cool, combined with closing up
the house during hot days and running the central air conditioner can be an
effective strategy.

The indoor thermostat should be set somewhere in the range of 22–25°C, depending
on your comfort requirements. A setting at the higher end of the range will
result in lower air-conditioning costs. If the humidity level is lower, temperature
settings can be at the higher end. Humidity levels can be reduced by using a
bathroom exhaust fan when you bathe or shower and by using a rangehood fan,
if it is vented outside, when cooking on the range top.

Continuous indoor fan operation can keep the temperature more uniform throughout
the house by eliminating temperature differences due to stratification. It can
also help keep the home cleaner, especially if there is an electronic air cleaner
installed.

However, continuous indoor fan operation can increase operating costs compared
with on-off or automatic fan operation. In more humid climates, the moisture
removed during compressor operation is re-evaporated by the fan operation when
the compressor is off. This can increase humidity levels and cause discomfort.

As in winter, adjusting the thermostat when the house is unoccupied can reduce
operating costs. If the house will be empty during the day, you can raise the
thermostat a few degrees before you leave and reset it to the preferred temperature
when you return. An automatic programmable thermostat will reliably adjust the
temperature for you to help you save money on cooling costs.

The power to the central unit should be shut off when the cooling season ends.
Most central air conditioners have a small electric heater on the compressor
to keep refrigerant out of the lubricating oil. Flip the circuit breaker to
turn this heater off. To prevent damage to the compressor, remember to turn
the power back on a day or two before you need to operate the central air conditioner.

Maintenance

Proper maintenance is critical in ensuring that your central air conditioner
will operate efficiently and have a long service life. You can do some of the
simple maintenance yourself, but you may also want to have a competent service
contractor do a periodic inspection of your unit. The best time to service a
central air conditioner is just prior to the cooling season.

Filter and coil maintenance can have a dramatic impact on system performance
and service life. Dirty filters and dirty indoor and outdoor coils and fans
reduce airflow through the system. This reduction in airflow decreases system
efficiency and capacity and can lead to expensive compressor damage if left
for an extended period of time.

Furnace filters should be inspected and cleaned or replaced, depending
on the type of furnace and the furnace manufacturer's instructions. The outdoor
coil should be vacuumed or brushed clean to keep it clear of dirt, leaves
and grass clippings. It can be carefully cleaned with a garden hose after debris
is vacuumed off. Consider a professional cleaning if the outdoor coil becomes
badly plugged.

Both the furnace fan and outdoor unit fan should be cleaned and lubricated
where applicable and following manufacturers' instructions. The furnace-fan
speed can be checked and adjusted at the same time, to ensure peak performance.

Ductwork can be professionally cleaned if needed, but the need for
cleaning can be reduced by a proper filter replacement and cleaning routine.
To ensure that all ducts are airtight, seal the joints with a special duct mastic
(sealant). This should reduce or eliminate air leaks. High temperature duct
tape may work, although it tends to degrade or permit air leaks over time. Be
sure that vents and registers are not blocked by furniture,
carpets or other items that can resist airflow. Extended periods of inadequate
airflow can lead to compressor damage. For professional cleaning or supplies,
look in the Yellow Pages™ under “Furnaces – Heating” or “Furnaces – Supplies
and Parts. ”

Using a high-efficiency air cleaner on a central cooling/heating system is
one way of ensuring a clean indoor coil and a cleaner indoor environment.

If, after attending to filter maintenance and coil cleaning, your central air
conditioner does not appear to be doing its job, you will need to hire a competent
service contractor to undertake more difficult maintenance or service, such
as checking the refrigerant level or making electrical or mechanical checks
and adjustments.

Operating costs

The operating cost of a central air conditioner is influenced by a number of
factors, such as how much you use your air conditioner and how efficient it
is, the amount of insulation and glazing in your home, and the frequency and
duration of door and window openings when the system is operating.

It also depends on the activities in your home and the use of other equipment
and appliances that increase the load on the air conditioner. Finally, it depends
on the local climate and electricity costs.

The section of his guide entitled “Air-Conditioning Operating
Costs, ” provides estimates of the cost of operating a central air conditioner
in different regions of Canada.

Life expectancy and warranties

The life expectancy of a central air conditioner is 15 years or longer. When
the air conditioner starts giving more problems than seem cost-effective to
fix – particularly when major components, such as a compressor, require replacement
– it may be time to replace the central air conditioner. New units offer greater
efficiency and lower operating costs; it may be more cost-effective in the long
run to replace rather than repair.

The warranty on your equipment will vary according to the manufacturer. Air
conditioner warranties range from one year for complete parts and labour to
five years for the compressor. Some manufacturers are now offering 10-year warranties
on their compressors. Make sure you fully understand the terms of a warranty.
Ask the contractor or manufacturer for an explanation, if necessary.

Replacing an existing central air conditioner

If your existing air conditioner needs replacement or is more than 10 years
old, chances are good that it is also inefficient. A 10-year-old air conditioner
probably has a SEER rating between 7.0 and 8.0, compared with some new models
that are twice as efficient. The more efficient unit should pay for itself through
decreased utility bills and offer improved reliability and warranty protection.

If you have an electric or oil furnace or a conventional gas furnace and your
space heating costs $1,000 or more per year, you should consider installing
an efficient air source or ground-source heat pump instead of a central air
conditioner. Find out from two or three contractors how much more it would cost
to add a heat pump to your furnace. Heat pump equipment SEER ratings are competitive
with hose of central air conditioners, but heat pumps have the added advantage
of providing savings in heating costs during the winter.

Heat pump savings range from a low of about 20 percent, where gas is the primary
heat source, to as high as 60 percent, where an electric furnace is the main
source of heating. If the additional capital cost divided by the estimated savings
is five years or less, consider installing a heat pump instead. They are good
for the environment and are an efficient way of using electricity for home heating.

For more information about heat pumps, read the companion NRCan booklet Heating
and Cooling with a Heat Pump. See “Need More Information?” to find
out how to order a copy.